A recent article published in The British Medical Journal (“The BMJ”) details the debate that has broken out over whether Pfizer should have told pregnant women participating in its trial of maternal respiratory syncytial virus (“RSV”) vaccination that a trial of GSK’s similar vaccine was halted over a safety signal around pre-term birth.
Pfizer’s maternal RSV vaccine is called Abrysvo.
Not only had GSK’s RSV vaccine trials shown a risk of babies being born prematurely but Pfizer was actively studying the risk as an “adverse event of special interest” in its trials.
The BMJ article described its investigation into the safety signal of increased pre-term and neonatal deaths and whether pregnant women who participated in Pfizer’s trial were adequately informed of the risk to their babies.
Recombinant RSV F protein vaccines are being developed by GlaxoSmithKline (“GSK”) and Pfizer to inoculate women during pregnancy. After safety concerns were raised: a potential increase in the risk for preterm births and neonatal deaths, GSK ceased its Phase 3 vaccine study on 28 February 2022.
GSK told The BMJ that the increase in preterm births was observed primarily in low and middle-income countries, driven particularly by South Africa. and not consistently after a peak in late 2021 and it was still investigating the cause of the pre-term births but was no longer developing its vaccine.
Pfizer was studying pre-term births as an adverse event of special interest in its own phase 3 trial, and a numerical (not statistically significant) increase in pre-term births has recently emerged in the data, although there are not enough data to understand if there is truly an increased risk or what the cause is.
Although 4 out of 14 members voted that the data presented by Pfizer were not adequate to support safety, the US Food and Drug Administration’s vaccines and related biological products advisory committee advised that Pfizer’s maternal RSV candidate was safe.
Committee member Paul Offit, professor of paediatrics at the Children’s Hospital of Philadelphia, said in a meeting that Pfizer’s and GSK’s vaccines were “almost identical” and so was concerned by GSK’s results.
Hana El Sahly, professor of molecular virology and microbiology at Baylor College of Medicine and committee chairwoman, said that the signal of increased preterm births connected to the Pfizer vaccine was “significant in the phase 2, in the phase 3, and in a very similar product,” adding that failing to design the Pfizer phase 3 study to deliver clarity was a “big missed opportunity.”